276 The Origin and Relations of the CarJ)on Minerals. 



dimiuntion, for two or three thousand years. We can only ac- 

 count for the persistence of this flow by supposing that it is 

 maintained by the gradual distillation of the carbonaceous 

 masses with which such evolutions of gas or of liquid hydro- 

 carbons are always connected. If it were true that carburetted. 

 hydrogen and petroleum are produced only from the primary 

 decomposition of organic tissue, it would be inevitable that at 

 least the elastic gases would have escaped long since. 



Oil-wells which have been nominally exhausted — that is, from 

 Avhicli the accumulations of centuries in rock reservoirs have been 

 pumped — and therefore have been abandoned, have in all cases 

 been found to be slowly replenished by a current and constant 

 secretion, apparently the product of an unceasing distillation. 



In the valley of the Cumberland, about Bnrkesville, one of 

 the oil-regions of the country, the gases escaping from the 

 equivalent of the Utica shale accumulate under the plates of 

 impervious limestone above, until masses of rock and. earth, 

 hundreds of tons in weight, are sometimes thrown out with great 

 violence. Unless these gases had been produced by compara 

 tively recent distillation, such ex})losions could not occur. 



In opening a coal-mine on a hillside, the first traces of the 

 coal-seam are found in a dark stain in the superficial clay ; then 

 a substance like rotten wood is reached, from which all the vola- 

 tile constituents have escaped. These appear, however, later, 

 and continue to increase as the mine is deepened, until under 

 water or a heaA'y covering of rock, the coal attains its normal 

 physical and chemical characters. Here it is evident that the 

 coal has undergone a long-continued distillation, which must 

 have resulted in the constant production of carbonic acid and 

 carburetted hydrogen. 



A line of perennial oil and gas springs marks the outcrop of 

 every great stratum of carbonaceous matter in the country. Of 

 these, the most considerable and remarkable are the bituminous 

 shales of the Silurian (Utica shale), of the Devonian (Hamilton 

 and Huron shales), the Carboniferous, etc. Here the carbona- 

 ceous constituent (10 to 20 per cent.) is disseminated through 

 a great proportion of inorganic material, clay and sand, and 

 seems both from the nature of the materials which furnished 

 it, — cellular plants and minute animal organisms, — and its dis- 



